Frequency-domain Modelling And Small-signal Stability Analysis For Multiple Grid-connected-converter System

With increasing penetration of renewable energy and power electronic in power system,modern power system gradually dominated by power electronic.Due to the negative resistance feature of power electronic device,a power system containing many power electronic devices is lack of inertia,since the small-signal stability of system face severe challenges.So,the frequency-domain modelling and small-signal stability analysis of a multi-grid-connected-converter system(MGCCS)is studied in this thesis.This thesis firstly deduces the impedance/admittance model of renewable energy grid-connected device based on linearization technique,and studies time-domain property and frequency-domain property for impedance/admittance model of grid-connected converter.Secondly,based on the impedance/admittance model of grid-connected equipment and transmission network,the equivalent negative feedback model of MGCCS in frequency-domain is derived,and the concept of generalized impedance ratio is proposed.In addition,the basic mathematical properties of the generalized impedance ratio function are discussed strictly in this thesis,which provides the necessary mathematical basis for small-signal stability analysis of MGCCS.In order to accurately analyze small-signal stability of MGCCS,this thesis first proposes a sufficient and necessary stability criterion with clear form based on the concept of equivalent open loop function,which realizes the accurate judgment and margin design of the system stability.Based on this criterion,the stability design of a MGCCS with tree topology is further simplified to the stability design of several subsystems.Finally,a case study of a grid-connected system with 3 convertor-groups is given.The results show that the proposed method can not only accurately judge the stability of the system,but also realize the decoupling optimization of a grid-connected system with tree topology.In order to reduce the complexity of small-signal stability analysis of MGCCS,a sufficient stability criterion with clear form and low complexity is proposed based on eigenvalue estimation of the generalized impedance ratio function.Based on the algebraic form of the criterion,the parameter sensitivity design of MGCCS is further proposed to assist the control parameter optimization design.Finally,a case study of a grid-connected system with 7 convertor-groups is given.The results show that the proposed method can not only conservatively judge the stability of the system,but also realize the quantitative optimization of the control parameters of the grid-connected system.